As Syria's President Bashar Assad clings mercilessly to power, hopes that his regime will be replaced by a stable, tolerant democracy are being dwarfed by fears of prolonged sectarian strife and Islamist radicalism. The outcome will hinge in part on a simple question: Whom do Syria's diverse rebels hate more, the United States or Iran?

The anomaly of power in modern Syria - where an Alawite minority rules over a Sunni Arab majority - was never sustainable, and few countries stand to lose more from the regime's collapse than the Islamic Republic of Iran. Syria has been Iran's only consistent ally since the 1979 revolution, providing the leadership in Tehran with a crucial thoroughfare to Iran's most important regional asset, the Lebanese Shiite militant group Hezbollah.

As a result, Iran has done its utmost to keep Assad afloat, providing billions of dollars of support as well as strategic aid to crush dissent. To relieve pressure on the Syrian military, Iran's Islamic Revolutionary Guards Corps is reportedly training two paramilitary organizations, Jaysh al Sha'abi and the Shabiha, which boast 50,000 fighters and are modeled on the Bassij militia that violently quashed Iran's 2009 popular uprisings.

This support can only delay, not prevent, Assad's demise. Thereafter Iran will face a strategic decision: whether to continue supporting a predominantly Alawite militia that represents only a small fraction of Syrian society, or to engage the Sunni Islamists who are poised to wield power in Damascus once Assad falls. Iran's leaders will try to embrace the Sunni radicals, and if that fails they will work with the Shabiha to prevent the formation of a stable, anti-Iranian order in Syria.

What's most important for Iran is not the sectarian makeup of Syria's future rulers, but a like-minded ideological worldview premised on resistance to the U.S. and Israel. As Iranian Supreme Leader Ali Khamenei once said, "We will support and help any nations, any groups fighting against the Zionist regime across the world." Iran's Sunni allies Hamas and Palestinian Islamic Jihad are cases in point.

Despite sharing common enemies with some Syrian rebels, there is no guarantee that Iran will be able to befriend the same forces it has helped to massacre over the past two years. Anti-Shiite, anti-Persian sentiment is rife among Syria's rebels, and the attraction of Iranian petrolargesse is eclipsed by the deeper pockets of Saudi Arabia and Qatar.

The question for the U.S. and allies such as Turkey is what can they do to ensure that moderate factions in the Syrian opposition come to dominate in a post-Assad Syria, and that they will prefer to work with the U.S. and its friends in the region, rather than with Iran.

That outcome isn't guaranteed, either. Iranian influence tends to thrive in countries suffering power vacuums and tumult, which they can attribute to U.S. or Israeli policies. They helped create Hezbollah after the 1982 Israeli invasion of civil-war era Lebanon. And in the aftermath of the 2003 U.S. invasion of Iraq, they helped entrench an Iraqi political class that is closer to Iran than the U.S. As Israel's minister of strategic affairs, Moshe Ya'alon, put it last year: "The Iranians know how to exploit every area and country that isn't properly governed."

This sordid history has made the Barack Obama administration reluctant to decisively enter the Syria fray, fearful of being sucked into an Islamist brier patch or another costly but fruitless exercise in nation-building.

Benign neglect, however, hasn't been so benign. Syria's humanitarian crisis has reached epic proportions, with more than 60,000 people killed and 2.5 million people displaced. The sense of abandonment and desperation felt by many Syrians has served to strengthen the most radical elements of the rebel forces, some of whom are thought to be aligned with al-Qaida.

Syria's hemorrhaging will continue to fuel radicalism until there is a change of political leadership in Damascus. In order to expedite this process, the U.S. administration must inhibit Iran's ability to arm and finance Assad.

This requires coercing the Iraqi government - the beneficiary of $2 billion in annual U.S. military aid - to halt the steady transit of Iranian military hardware and personnel to Syria. It also means making clear to Lebanon that it must curtail Hezbollah's cross-border operations into Syria, and ensure that Iran can't use Lebanese banks to evade international sanctions. The U.S. and its allies should expose the governments of both countries as abettors to Assad's criminal regime, should they continue to be complicit in Iran's operations.

Concurrently, the U.S. and like-minded allies such as Turkey should strengthen the competence and cohesiveness of the National Coalition for Syrian Revolutionary and Opposition Forces, the exiled body they helped shape. This might encourage reluctant fence sitters to abandon Assad's ship, shortening the duration of what will otherwise be an increasingly sectarian conflict.

Aid channeled through the opposition council, in coordination with credible international nonprofit organizations, should provide for millions who have been displaced. Not doing so risks leaving Iran and radical Sunni Islamists to exploit human suffering for recruitment purposes.

A nascent rebel joint military command, working under the opposition council, would allow anti-regime forces to better coordinate operations and steer fighters away from jihadi ideology, and could lay the foundation for a future national army. Military assistance, direct from the U.S. or through regional allies, must flow through this joint command. Doing so will afford moderates a better chance to succeed against both the Iranian-backed militias and the growing numbers of Sunni jihadists who are fighting in Syria.

A greater U.S. role won't render Syria an American-allied democracy. That possibility, if it ever existed, has long been lost. But continued U.S. inaction risks leaving Syria at the mercy of Iran and Sunni extremists whose intolerance, and hatred of the U.S., dwarfs any concerns they may have for the well- being of Syria and its people. Such an outcome would haunt Syria, the Middle East and the U.S. for years to come.

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Karim Sadjadpour is a senior associate at the Carnegie Endowment for International Peace. Firas Maksad is director of New Policy Advisors, a Washington advisory group.

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